The invention is relates to a method of measuring operating data of internal combustion engines. Ion current probes are known which serve, for example, to determine and control the point in time of ignition in Otto-motors (DE-OS No. 29 35 725) or for picking up pressure fluctuations to recognize knocking combustions (DE-OS No. 28 02 202). These probes operate with high field intensities of over 1000 V/cm and are not suitable for determining harmful materials in the combustion gas, because of their other features.